Not sure if you've experienced an issue where certain services don't actually play nicely with GTM-Oauth2 -- the ones I had issues with were Facebook and Github -- but a guy just said he patched it (haven't tested it myself yet though).
Thread of the issue can be seen here http://groups.google.com/group/gtm-oauth2/browse_thread/thread/2c0ae0bc9f3a8dfc
Link to patch is here http://code.google.com/p/gtm-oauth2/issues/detail?id=1
Not sure if that's helpful to you at all, just wanted to give you a heads up though in case it is!